MS 909 - Conjugated Dienes and the Allyl Position

Submitted by Matt on August 6, 2011.

Textbook and Chapter: Carey and Giuliano 8th Ed. (2010), Chapter 10

Keywords: allylic position, conjugate addition

Description: Covers topics related to the allyl position and conjugated dienes:

Allyl cation, anion, and radical resonance forms
What conjugated dienes can or can't undergo Diels-Alder reactions
Allylic brominations using N-bromosuccinimide (NBS)
1,2 and 1,4 additions (thermodynamic vs kinetic conditions)

Total Problems: 4

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Problem # 569

Draw all resonance forms for each species.

For the anion and cation species, used curved arrows. For the radical species, use hooks.
Solution
Notice that arrows always go from high electron density to low electron density. So for negatively charged species, the arrow starts at the lone pair. For positively charged species, the arrow starts at the double bond and goes towards the positive charge.

(Radical resonance uses hooks instead of arrows and so is a little different.)

Problem Details
MendelSet practice problem # 569 submitted by Matt on July 8, 2011.
Subject: Organic Chemistry
Subject Topic(s): Lewis Structures, Formal Charges, and Resonance, Conjugated Dienes and the Allylic Position (Kinetic vs Thermodynamic Control)
Question Type: Concept (explain why this is so..)
Keyword(s): resonance, allylic position
Problem # 571

Write the structure of the major organic product of each reaction.
Solution
a) is a free-radical bromination. The radical will be formed at the most stable position, which will be the allylic position due to resonance (see problem 569). There are two allylic positions in this molecule, so the more substituted one will be the more stable; the bromine will add to the 3º allylic carbon.

b) is an E2 reaction. There are two types of beta protons and so two possible alkene products. One is an isolated diene and the other is a conjugated diene. Conjugated dienes are more stable, and so that will be the major product.

Problem Details
MendelSet practice problem # 571 submitted by Matt on July 8, 2011.
Subject: Organic Chemistry
Subject Topic(s): Conjugated Dienes and the Allylic Position (Kinetic vs Thermodynamic Control)
Question Type: Concept (explain why this is so..)
Keyword(s): allylic position
Problem # 572

Let's work through a conjugate addition problem.

Write out the mechnanism for the formation of the 1,2 and 1,4 products of the reaction below.
Solution
This is an S_N1 reaction, but the carbocation is special because it's allylic.

If the Br^- nucleophile attacks the 3º resonance form, the 1,2 product is formed. (This is the product that would have formed if resonance was not involved.)

If the Br^- nucleophile attacks the 2º resonance form, the 1,4 product is formed.

Problem Details
MendelSet practice problem # 572 submitted by Matt on July 8, 2011.
Subject: Organic Chemistry
Subject Topic(s): Conjugated Dienes and the Allylic Position (Kinetic vs Thermodynamic Control)
Question Type: Mechanism (show a mechanism using curved arrows..)
Keyword(s): SN1 mechanism, conjugate addition, allylic position
Problem # 522

Let's work through a 1,2 and 1,4 addition. Draw the structures for each of the species in the six boxes below. Also draw curved arrows to show electron movement.
Solution
This is an S_N1 reaction, but with a twist.

The top reaction is a regular S_N1 reaction; the leaving group (Br^-) leaves and the nucleophile (CH₃OH) attacks the carbocation.

The twist is that the carbocation is allylic, and has resonance, so there is another carbon that the nucleophile can attack.

So instead of just one S_N1 product, two products are formed. The product that doesn't involve resonance is called the direct addition or 1,2 product. The product that involves allylic resonance is called the conjugate addition or 1,4 product.

Problem Details
MendelSet practice problem # 522 submitted by Matt on July 1, 2011.
Subject: Organic Chemistry
Subject Topic(s): Conjugated Dienes and the Allylic Position (Kinetic vs Thermodynamic Control)
Question Type: Concept (explain why this is so..)
Keyword(s): SN1 mechanism, conjugate addition

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MS 909 - Conjugated Dienes and the Allyl Position

MS 909 - Conjugated Dienes and the Allyl Position

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